The present invention relates to fuel oil compositions, processes for their preparation, and their use in compression-ignition engines.
As stated in WO 95 33805 (Exxon) environmental concerns have led to a need for fuels with reduced sulphur content, especially diesel fuel and kerosene. However, the refining processes that produce fuels with low sulphur contents also result in a product of lower viscosity and a lower content of other components in the fuel that contribute to its lubricity, for example, polycyclic aromatics and polar compounds. Furthermore, sulphur-containing compounds in general are regarded as providing anti-wear properties and a result of the reduction in their proportions, together with the reduction in proportions of other components providing lubricity, has been an increase in reported failures of fuel pumps in diesel engines using low-sulphur fuels, the failure being caused by wear in, for example, cam plates, rollers, spindles and drive shafts.
This problem may be expected to become worse in future because, in order to meet stricter requirements on exhaust emissions generally, high pressure fuel pumps, for example rotary and unit injector systems, are being introduced, these being expected to have more stringent lubricity requirements than present equipment, at the same time as lower sulphur levels in fuels become more widely required.
At present, a typical sulphur content in a diesel fuel is about 0.25% by weight (2500 ppmw). In Europe maximum sulphur levels have been reduced to 0.05% (500 ppmw), in Sweden grades of fuel with levels below 0.005% (50 ppmw) (Class 2) and 0.001% (10 ppmw) (Class 1) are already being introduced.
In the context of this specification low-sulphur fuels are those having a sulphur level below 0.05% by weight (500 ppmw)
Many additives have been described over the years for enhancing engine cleanliness, e.g. for reducing or removing deposits in the intake system (e.g. carburetors, intake manifold, inlet valves) or combustion chamber surfaces of spark-ignition engines, or for reducing or preventing injector nozzle fouling in compression-ignition engines
For example, UK Patent Specification No. 960,493. (California Research Corporation) published Jun. 10, 1964, describes the incorporation of metal-free detergents, in the form of polyolefin substituted succinimides of tetraethylene pentamine, in base fuels for internal combustion engines. The succinimides disclosed correspond to the general formula: 
wherein R is derived from a polymer RH of an olefin containing from 2 to 5 carbon atoms, which polymer contains from 30 to 200 carbon atoms. The molecular weight of the radical R is said to range from 400 to 3000, more preferably, 900 to 1200 and is advantageously derived from a polymer of isobutene having a molecular weight of about 1000. The single example of the preparation of such a succinimide is based on polysibotuylene of molecular weight about 1000, and tests are described using the resulting succinimide in gasoline and in a high-sulphur diesel fuel (sulphur content of 0.5% w, i.e. 5,000 ppmw).
More recent publications, e.g. those dating from after expiry of UK Patent No. 960,493 and its equivalents, teach the use of somewhat different succinimides and other succinic acid derivatives in fuel compositions
Thus, EP-B-147 240 (Ethyl) describes a distillate fuel composition for indirect injection compression ignition engines containing in an amount sufficient to suppress and preferably to minimise coking in nozzles of indirect injection compression ignition engines operated on such fuel a combination of (a) organic nitrate ignition accelerator and (b) hydrocarbyl-substituted succinimide or succinamide, and optionally: (c) hydrocarbyl amine having from 3 to 60 carbons and from 1 to 10 nitrogens, or a combination of the hydrocarbyl amine (c) and (d) N,Nxe2x80x2-disalicylidene-1,2-diaminopropane. The hydrocarbyl-substituted succinimide is preferably an olefin-polymer substituted succinimide wherein the olefin polymer substituent has an average molecular weight of 500-500,000, preferably being a polyisobutene substituent having an average molecular weight of 700-5,000. The succinimide portion is preferably derived from a polyalkyleneamine of formula H2Nxe2x80x94(Rxe2x80x94NH)nxe2x80x94H wherein R is a divalent aliphatic hydrocarbon having 2 to 4 carbon atoms and n is an integer from 1 to 10, including mixtures thereof, and the polyalkyleneamine is preferably a polyethyleneamine having 2 to 6 ethylene units. The most preferred succinimide-succinamide component is the commercial product xe2x80x9cHITEC E-644xe2x80x9d (trade mark), which is used in the examples and is described as being xe2x80x9cmade by reacting two moles of a polyisobutenyl succinic anhydride with one mole of a polyethylene amine mixture having an average composition corresponding to tetraethylene pentaminexe2x80x9d (Page 7, lines 4 to 6). The base fuel used in the examples was a high-sulphur fuel (sulphur content of 0.41%w, i.e. 4,100 ppmw).
EP-A-482 253 (Ethyl) describes a fuel composition which comprises a liquid middle distillate hydrocarbonaceous fuel containing at least one fuel-soluble ashless dispersant in an amount of at least 50 ppm sufficient to cause a prompt reduction in emissions released upon combustion of said fuel composition. In the example, the ashless dispersant is described as xe2x80x9ca polyisobutenyl succinimide of tetraethylene pentamine in which the number average molecular weight of the polyisobutenyl group is about 950, used as a 75% solution in high aromatic solventxe2x80x9d (Page 10, lines 11 to 13), and the base fuel was a high-sulphur fuel (sulphur content of 0.125%w (Page 10, line 27), i.e. 1,250 ppmw). The general description relating to succinimides of ethylene polyamines including tetraethylene pentamine simply states xe2x80x9cthese ethylene polyamines have a primary amine group at each end so can form mono-alkenylsuccinimides and bis-alkenylsuccinimidesxe2x80x9d (Page 3, lines 9 and 10). No distinction or preference is expressed as between mono- and bis-succinimides.
EP-A-613 938 (BP) describes hydrocarbon fuel compositions comprising a hydrocarbon fuel and a hydrocarbyl succinic diamide derived from a secondary amine. The hydrocarbon fuel xe2x80x9cmay suitably comprise a hydrocarbon fraction boiling in the gasoline range or a hydrocarbon fraction boiling in the diesel rangexe2x80x9d (Page 5 lines 10 and 11). There is no discussion of sulphur content. The engine tests in the examples are in an Open Kadett engine (clearly a spark-ignition engine) and as comparative examples there are used (1) a mono-succinimide derived from a polyisobutylene succinic anhydride of PIB Mn about, 1000 and tetraethylene pentamine and (2) a bis-succinimide derived from the same polyisobutylene succinic anhydride and triethylene tetramine. The mono-succinimide gave results which were significantly inferior to the bis-succinimide (deposits mg/valve of 229 compared with 40, valve rating of 8.0 compared with 9.70) (Page 9, Table 2).
EP-B-557 561 (Chevron) discloses fuel compositions comprising a major amount of hydrocarbons boiling in the gasoline or diesel range and an effective detergent amount of an additive composition comprising:
(a) a polyisobutenyl succinimide having the formula 
wherein R is a polyisobutenyl group having a number average molecular weight in the range of 1200 to 1500, preferably 1200 to 1400, more preferably 1250 to 1350, and most preferably about 1300, and x is 1 or 2, and (b) a nonvolatile paraffinic or naphthenic carrier oil, or a mixture thereof. It is stated (Page 3, lines 8 to 12) that the invention therein xe2x80x9cis based on the surprising discovery that the unique combination of a polyisobutenyl succinimide and a carrier oil, wherein the polyisobutenyl succinimide is derived from ethylenediamine or diethylenetriamine and the polyisobutenyl group has an average molecular weight of 1200 to 1500, provides unexpectedly superior deposit control performance when compared to prior art polyisobutenyl succinimides of lower molecular weightxe2x80x9d. There is no discussion of sulphur content of the fuel. The deposit control tests in the examples are in gasoline, and as comparative examples there are described monosuccinimides (PIB Mn 950) of ethylene-diamine (Example 1) and of diethylenetriamine (Example 2). These are shown to give significantly inferior intake value deposit weights (Average 127.9 and 105.2 mg, respectively) compared with the directly equivalent materials derived from PIB of Mn 1300 (Average 72.2 and 35.1 mg, respectively).
U.S. Pat. No. 5,478,367 (ass. Exxon) discloses diesel fuel compositions containing certain macrocyclic polyamines derived from reaction of polyisobutylene succinic anhydride with certain polyamines, for reduction of particulate emissions on combustion. The diesel fuel used in the examples was a high-sulphur fuel (sulphur content of 0.23%w, i.e. 2,300 ppmw) The macrocyclic polyamines were characterised by their IR spectra having four peaks between 1900 and 1500 wave numbers This is contrasted with the IR spectra of uncyclised equivalents, which only have three peaks in this region (Column 9, lines 16 to 22).
WO 94 20593 (Mobil) discloses a low emission diesel fuel having a cetane number in the range of 50 to 60, which comprises:
(i) a straight run hydrocarbon distillate having an initial boiling point in the range of 170xc2x0 C. to 190xc2x0 C., an end point not higher than 315xc2x0 C., a sulphur content of less than 0.1 wt percent (preferably sulphur content from 0.005 to 0.05%w, i.e. 50 to 500 ppmw) and aromatics content of 18 to 30 wt. percent, a maximum specific gravity of 0.83 at 15xc2x0 C. and an API gravity of 38 to 43, and
(ii) an additive package comprising a detergent, a friction reducing additive and a cetane number improver. In the examples, two base fuels are used, one of sulphur content 0.01%w (100 ppmw) and one of 0.06%w (600 ppmw). The polyisobutylene succinimide detergent used in the examples is not specifically identified, but the general description does specifically state (Page 6, lines 10 to 14) that a xe2x80x9cpreferred detergent is a polybutenyl bis(succinimide) produced from a polybutenyl succinic anhydride and tetraethylene pentamine (2:1 ratio, pb mol. wt. about 1200) in combination with ethylene diamine tetraacetic acidxe2x80x9d.
WO 9623855 (Exxon) discusses in detail the lubricity problems of low-sulphur diesel fuels and discloses a fuel oil composition comprising a major amount of a fuel oil containing not more than 0.05% by weight of sulphur (i.e. 500 ppmw) and having a 95% distillation point of not greater than 350xc2x0 C., and a minor amount of an additive composition comprising:
(a) an ashless dispersant comprising an acylated nitrogen compound, and
(b) a carboxylic acid, or an ester of the carboxylic acid and an alcohol wherein the acid has from 2 to 50 carbon atoms and the alcohol has one or more carbon atoms. Preferred acylated nitrogen compounds are those made by reacting a polyisobutenyl succinic anhydride with mixtures of ethylene polyamines. In the examples, the ashless dispersant employed is described as a xe2x80x9csuccinimide ashless dispersant being the reaction product of 1.5 equivalents of PIBSA (polyisobutyl succinic anhydride, with polyisobutylene number average molecular weight of approximately 950, as measured by Gel Permeation Chromatography) with one equivalent of polyethylene polyanine mixture of average composition approximating to pentaethylene hexaminexe2x80x9d. High frequency reciprocating rig test data are given for this ashless dispersant in a low-sulphur diesel fuel of sulphur content 0.05%w (500 ppmw) together with, as (b), a xe2x80x9creaction product of equimolar amounts of ethylene glycol and dilinoleic acid, subsequently reacted with methanol, being a mixture of estersxe2x80x9d, and in a low-sulphur diesel fuel of sulphur content 0.03% w (300 ppmw ) together with this same reaction product and, as an alternative (b), with xe2x80x9ca commercial mixture of dimer fatty acids, predominantly dilinoleic acidxe2x80x9d. Further examples employ a low-sulphur diesel fuel of sulphur content 0.00045% (4.5 ppmw) and are filterability tests using this ashless dispersant together with, as (b), ether a sorbitan mono-oleate ester or a glycerol mono-oleate ester.
There are numerous patents relating to lubricity improvers for low-sulphur diesel fuels, developed in the light of the paper by Danping Wei and H. A. Spikes. xe2x80x9cThe Lubricity of Diesel Fuelsxe2x80x9d. Wear, III(1986) 217-235, for example those following.
WO 95 33805 (Exxon) describes the use of cold flow improvers to enhance lubricity of lowy-sulphur fuels.
WO 94 17160 (Exxon) describes the use of certain esters of a carboxyclic acid and an alcohol wherein the acid has from 2 to 50 carbon atoms and the alcohol has one or more carbon atoms, particularly glycerol monooleate and di-isodecyl adipate, as additives for fuel oils for wear reduction in the injection system of a compression-ignition engine.
U.S. Pat. No. 5,484,462 (Texaco) mentions dimerized linoleic acid as a commercially available lubricity agent for low sulphur diesel fuel (Col. 1, line 38), and itself provides aminoalkylmorpholines as fuel lubricity improvers.
U.S. Pat. No. 5,490,864 (Texaco) describes certain dithiophosphoric diester-dialcohols as anti-wear lubricity additives for low-sulphur diesel fuels.
The present Applicant""s European Patent Application No 96304975.4, filed Jul. 5, 1996, (Applicant""s ref. TS 7520 EPC) discloses that certain alkyl aromatic compounds having at least one carboxyl group attached to their aromatic nuclei can confer anti-wear lubricity effects when incorporated in fuel oil, especially low-sulphur diesel fuel. The alkyl aromatic compounds are those wherein at least one alkyl group of 6 to 30 carbon atoms is attached to an aromatic nucleus and at least one carboxyl group and optionally one or two hydroxyl groups are attached to the aromatic nucleus Preferred alkyl aromatic compounds are alkyl benzoic acids or alkylsalicylic acids containing one or two alkyl groups, preferably of 6 to 30 carbon atoms, more preferably a C8-20 alkyl group, advantageously a C8-18 alkyl group.
It has now surprisingly been found that use of a certain narrow band of dispersant additives in low-sulfur diesel fuel are dramatically more effective in enhancing, injector cleanliness than closely analogous dispersant additives. Such dramatic differences are not observed and are not predictable when the various dispersant additives are used in conventional, high-sulphur, diesel fuels.
According to the present invention there is provided a fuel oil composition comprising a major proportion of a liquid hydrocarbon middle distillate fuel oil having a sulphur concentration of at most 0.05% by weight, and a minor proportion of a dispersant additive obtained by reacting, in a molar ratio A:B in the range 4:3 to 1:10. (A) a polyalkenyl derivative of monoethylenically unsaturated C4-C10 dicarboxylic acid material in which the number average molecular weight (Mn) of the polyalkenyl chain is in the range from 850 to 1150 with (B) a polyamine of general formula
H2N(CH2)mxe2x80x94[NH(CH2)m]nxe2x80x94NH2xe2x80x83xe2x80x83(1)
where m is in the range from 2 to 4 and n is in the range from 1 to 6.
In another aspect, the present invention provides a fuel oil composition comprising a major proportion of a liquid hydrocarbon middle distillate fuel oil having a sulphur concentration of at most 0.05% by weight and a minor amount of an additive composition comprising a dispersant additive and a lubricity additive, wherein the dispersant additive is obtained by reacting, in a molar ratio A:B in the range 4:3 to 1:10. (A) a polyalkenyl derivative of monoethylenically unsaturated C4-C10 dicarboxylic acid material in which the number average molecular weight (Mn) of the polyalkenyl chain is in the range from 850 to 1150 with (B) a polyamine of general formula
H2N(CH2)mxe2x80x94[NH(CH2)m]nxe2x80x94NH2xe2x80x83xe2x80x83(1)
where m is in the range from 2 to 4 and n is in the range from 1 to 6.
The present invention provides a fuel oil composition comprising, a major proportion of a liquid hydrocarbon middle distillate fuel oil having a sulphur concentration of at most 0.05% by weight, and a minor proportion of a dispersant additive obtained by reacting, in a molar ratio A:B in the range 4:3 to 1:10. (A) a polyalkenyl derivative of monoethylenically unsaturated C4-C10 dicarboxylic acid material in which the number average molecular weight (Mn) of the polyalkenyl chain is in the range from 850 to 1150 with (B) a polyamine of general formula
H2N(CH2)mxe2x80x94[NH(CH2)m]nxe2x80x94NH2xe2x80x83xe2x80x83(1)
where m is in the range from 2 to 4 and n is in the range from 1 to 6. The present invention further provides a fuel oil composition comprising a major proportion of a liquid hydrocarbon middle distillate fuel oil having a sulphur concentration of at most 0.05% by weight and a minor amount of an additive composition comprising a dispersant additive and a lubricity additive, wherein the dispersant additive is obtained by reacting, in a molar ratio A:B in the range 4.3 to 1:10. (A) a polyalkenyl derivative of monoethylenically unsaturated C4-C10 dicarboxylic acid material in which the number average molecular weight (Mn) of the polyalkenyl chain is in the range from 850 to 1150 with (B) a polyamine of general formula
H2N(CH2)mxe2x80x94[NH(CH2)m]nxe2x80x94NH2xe2x80x83xe2x80x83(1)
where m is in the range from 2 to 4 and n is in the range from 1 to 6.
The middle distillate fuel oil is derived from petroleum and will typically have a boiling range in the range 100xc2x0 C. to 500xc2x0 C. e.g. 150xc2x0 C. to 400xc2x0 C. Such petroleum-derived fuel oils may comprise atmospheric distillate or vacuum distillate, or cracked gas oil or a blend in any proportion of straight run and thermally and/or catalytically cracked distillates. Preferred fuel oil compositions of the invention are diesel fuel compositions. Diesel fuels typically have initial distillation temperature about 160xc2x0 C. and final distillation temperature of 290-360xc2x0 C., depending on fuel grade and use.
The fuel oil itself may be an additised (additive-containing) oil or an unadditised (additive-free) oil. If the fuel oil is an additised oil, it will contain minor amounts of one or more additives e.g. one or more additives selected from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers) and wax anti-settling agents (e.g. those commercially available under the Trade Marks xe2x80x9cPARAFLOWxe2x80x9d (e.g. xe2x80x9cPARAFLOWxe2x80x9d 450; ex Paramins). xe2x80x9cOCTELxe2x80x9d (e.g. xe2x80x9cOCTELxe2x80x9d W 5000; ex Octel) and xe2x80x9cDODIFLOWxe2x80x9d (e.g. DODIFLOWxe2x80x9d v 3958; ex Hoechst).
The fuel oil has a sulphur content of at most 0.05% by weight (500 ppmw) (xe2x80x9cppmwxe2x80x9d is parts per million by weight) Advantageous compositions of the invention are also attained when the sulphur content of the fuel oil is below 0.005% by weight (50 ppmw) or even below 0.001% by weight (10 ppmw).
The polyalkenyl derivatives of monoethylenically unsaturated C4-C10 dicarboxylic acid material are known compounds or can be prepared by processes analogous to known processes Thus, such a derivative may conveniently be prepared by mixing a polyalkene with a specified amount of a monoethylenically unsaturated C4-C10 dicarboxylic acid material and passing chlorine through the mixture, e.g. as described in GB-A-949,981. Alternatively, the derivative may be prepared by reacting thermally at an appropriate temperature the polyalkene with a specified amount of the dicarboxylic acid material, e.g. as described in GB-A-1,483,729. A particularly advantageous process for preparing the derivative involves reacting the polyalkene with the dicarboxylic acid material in mol ratio dicarboxylic acid material polyalkene greater than 1:1, at a temperature in the range 150 to 260xc2x0 C., if desired in the presence of a polyaddition-inhibiting amount of a sulphonic acid, as described in EP-A-542,380.
The polyalkene may conveniently be a homopolymer or copolymer, for example of at least one C2-C10 monoolefin. Preferably the polyalkene is a polymer of at least one C2-C5 monoolefin, e.g. an ethylene-propylene copolymer. The monoolefin is preferably a C3-C4 olefin and preferred polyalkenes derived therefrom include polyisobutylenes and atactic or isotactic propylene oligomers.
The monoolefin is most preferably isobutylene, so polyisobutylenes are the preferred form of polyalkene. Examples of suitable commercial polyisobutylenes are those sold by BP under the trade marks xe2x80x9cHYVIS 10xe2x80x9d, xe2x80x9cNAPVIS 10xe2x80x9d and xe2x80x9cULTRAVIS 10xe2x80x9d, that sold by Exxon under the trade mark xe2x80x9cPARAPOL 950xe2x80x9d, that sold by BASF under the trade mark xe2x80x9cGLISSOPAL 1000xe2x80x9d and that sold by Amoco under the trade mark xe2x80x9cINDOPOL H 100xe2x80x9d.
The number average molecular weight, Mn, of polyalkenes may be determined by several techniques which give closely similar results. Conveniently, Mn may be determined for examples by modern gel permeation chromatography (GPC), e.g. as described for example in W. W. Yau, J. J. Kirkland and D. D. Bly, xe2x80x9cModern Size Exclusion Liquid Chromatographyxe2x80x9d. John Wiley and Sons, New York. 1979.
The number average molecular freight of the polyalkenyl chain is in the range from 850 to 1150, preferably 850 to 1000.
C4-C10 dicarboxylic acid materials (see for example U.S. Pat. Nos. 4,086,51 and 4,235,786) may for example be anhydrides, e.g. of C4-C6 dicarboxylic acids such as maleic acid, citraconic acid (methylmaleic acid), itaconic acid (methylene succinic acid) and ethylmaleic acid. The C4-C10 dicarboxylic acid material is preferably maleic anhydride.
When the C4-C10 dicarboxylic acid material is maleic anhydride, the polyalkenyl derivative will be a polyalkenyl succinic acid derivative.
The ratio of dicarboxylic acid moieties per polyalkenyl chain (referred to as xe2x80x9csuccination ratioxe2x80x9d when the dicarboxylic acid material is maleic anhydride), r, may readily be determined by a procedure which will be described later, in the examples, xe2x80x9crxe2x80x9d is preferably not greater than 1 2:1
Examples of polyamines of formula 1 above include diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, hexaethylene heptamine, tripropylene tetramine, and corresponding commercial mixtures such as xe2x80x9cPolyamine Hxe2x80x9d, xe2x80x9cPolyamine 400xe2x80x9d and xe2x80x9cPolyamine Sxe2x80x9d. Preferably n is in the range 1 to 3. Preferably m is 2, so the preferred polyamines are polyethyleneamines.
The molar ratio A:B is preferably less than 6:5, preferably in the range 6:5 to 1:2. Where excess polyamine is employed, unreacted amine may conveniently be removed by evaporation or by washing, e.g. with aqueous medium such as a methanol/water mixture.
The dispersant additive are generally mixtures of amide and imide species, as demonstrated by infra-red spectroscopy.
The dispersant additive is preferably present in an amount in the range of from 10 to 400 ppmw, more preferably 40 to 200 ppmw, active matter based on total composition.
Fuel oil compositions of the present invention preferably additionally contain a lubricity additive in an amount in the range from 50 to 500 ppmw based on total composition. The lubricity additive may be any lubricity additive, e.g. as described above. Commercial lubricity additives include those available as EC 831 (ex Paramins). xe2x80x9cHITECxe2x80x9d (trade mark) E580 (ex Ethyl Corporation) and xe2x80x9cPARADYNExe2x80x9d (trade mark) 655 (ex Exxon Chemical Ltd).
The present invention further provides a process for the preparation of a fuel oil composition according to the invention, as defined above, which comprises admixing the dispersant additive or an additive concentrate containing the dispersant additive with the fuel oil.
Additive concentrates suitable for incorporating in the fuel oil compositions will contain the dispersant additive, preferably together with the lubricity additive, and a fuel-compatible diluent, which may be a carrier oil (e.g. a mineral oil), a polyether, which may be capped or uncapped, a non-polar solvent such as toluene, xylene, white spirits and those sold by member companies of the Royal Dutch/Shell Group under the Trade Mark xe2x80x9cSHELLSOLxe2x80x9d, and/or a polar solvent such as esters and, in particular, alcohols, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by member companies of the Royal Dutch/Shell Group under the Trade Mark xe2x80x9cLINEVOLxe2x80x9d, especially xe2x80x9cLINEVOLxe2x80x9d 79 alcohol which is a mixture of C7-9 primary alcohols, or the C12-14 alcohol mixture commercially available from Sidobre Sinnova, France under the Trade Mark xe2x80x9cSIPOLxe2x80x9d.
Additive concentrates and fuel oil compositions prepared therefrom may further contain additional additives such as dehazers, e.g. alkoxylated phenol formaldehyde polymers such as those commercially available as xe2x80x9cNALCOxe2x80x9d (trade mark) EC5462A (formerly 7D07) (ex Nalco), and xe2x80x9cTOLADxe2x80x9d (trade mark) 2683 (ex Petrolite); anti-foaming agents (e.g. the polyether-modified polysiloxanes commercially available as xe2x80x9cTEGOPRENxe2x80x9d (trade mark) 5851, Q 25907 (ex Dow Corning), xe2x80x9cSAGxe2x80x9d (trade mark) TP-325 (ex OSi), or xe2x80x9cRHODORSILxe2x80x9d (trade mark) (ex Rhone Poulene)); ignition improvers (e.g. 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tertiarybutyl peroxide and those disclosed in U.S. Pat. No. 4,208,190 at Column 2, line 27 to Column 3, line 21); anti-rust agents (e.g. that commercially sold by Rhein Chemie, Mannheim, Germany as xe2x80x9cRC 4801xe2x80x9d, a propane-1,2-diol semiester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid), reodorants, anti-wear additives; anti-oxidants (e.g. phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,Nxe2x80x2-di-sec-butyl-p-phenylenediamine), and metal deactivators.
Unless otherwise stated, the (active matter) concentration of each additional additive in the diesel fuel is preferably up to 1 percent by weight, more preferably in the range from 5 to 1000 ppmw (parts per million by weight of the diesel fuel), advantageously 75 to 300 ppmw e.g. 95 to 150 ppmw.
The (active matter) concentration of the dehazer in the diesel fuel is preferably in the range from 1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw and advantageously from 1 to 5 ppmw. The (active matter) concentrations of other additives (with the exception of the ignition improver) are each preferably in the range from 0 to 20 ppmw, more preferably from 0 to 10 ppmw. The (active matter) concentration of the ignition improver in the diesel fuel is preferably in the range from 0 to 600 ppmw and more preferably from 0 to 500 ppmw. If an ignition improver is incorporated into the diesel fuel, it may conveniently be used in an amount of 300 to 500 ppmw.
The invention further provides a method of operating a compression-ignition engine which comprises introducing into the combustion chambers of said engine a fuel composition according to the invention as defined above.